Propellant performance is determined from its ability to convert chemical energy into mechanical energy through the evolution of heat and gases that apply pressure to the base of a projectile moving it down the bore of a barrel. Many factors influence this process. Chemical composition is one important characteristic and another is grain morphology (shape and size) which has a profound effect on the burning rate. To arrive at an optimised propellant design it must be understood that the materials, processing conditions, physical properties and chemical properties are all interlinked to determine propellant performance. The goal is to achieve efficient combustion with optimised loadability to deliver improved ballistic performance. In addition, other aspects such as improving shelf life of the propellant or ensuring ballistic consistency over temperature extremes are also important. It is also recognized that new propellant formulations and production processes are required in order to improve efficiency and meet more stringent safety, toxicity and environmental impact requirements.
To improve propellant performance, and to prevent dangerously high pressure build up, a burn deterrent (or burn rate modifier) is typically added to the propellant to regulate the burn rate in the initial part of the ballistic process. This is typically achieved by coating a chemical onto a propellant grain. The chemical can penetrate to some extent into the grain matrix and acts to slow the burning reaction (by interrupting the chain reaction of burning) or the chemical is cooler burning. Burn deterrents that function by interrupting the chain reaction of burning do so by stabilising free radicals. This stabilisation extends the lifetime of the radicals, slows the rate of the radical processes and subsequently, there is less, or slower, combustion.
An example of a burn rate deterrent is dinitrotoluene (DNT). DNT is an effective burn deterrent because it is relatively easy to apply, stable over long periods and is chemically compatible with materials such as nitrocellulose which is the major energetic component of most small arms propellants. However, it is highly toxic and a suspected carcinogen which makes it a chemical of concern. Recent legislation (such as Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) under the European Union) has resulted in the use of DNT being highly regulated with the potential for DNT to be banned in Europe. Due to its characteristics, DNT has associated environmental problems in that it builds up in and around factory buildings, migrates very slowly into the soil and breaks down slowly.
Other currently available burn rate modifiers, such as dibutylphthalate (DBP), are also on the substance of concern list and are likely to be banned. It is anticipated that materials such as DNT and DBP will also have tighter restriction applied as other countries adopt more stringent safety and environmental regulations.
There therefore exists a need for alternative burn rate modifier to DNT and other burn rate modifiers currently in use.